Differential mobility spectrometry separates ions based upon the difference between high and low field mobility, typically at or near atmospheric pressure. Ions drift through a mobility cell, typically having two electrodes separated by a substantially uniform gap, and are separated by exposure to alternating high and low electric field conditions. The separation field is controlled by application of an asymmetric waveform to electrodes within the mobility cell. Depending on the difference between the high-field and low-field mobility of an ion, it will migrate toward one or the other electrode. A small DC field can be applied between the electrodes to steer ions back to the central axis of the mobility cell such that they may be transmitted to a downstream detector, or instrument such as a mass spectrometer. Only ions with specific differential mobility will pass through the device.
The dominant analyzer geometries that are used today are characterized by either flat planar electrodes providing a homogeneous electric field, or curved cell geometries that create inhomogeneous fields. The former is popularly referred to as a differential mobility spectrometer (DMS), and the latter is referred to as High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) (collectively referred to herein as ion mobility spectrometers). The present teachings will be described in conjunction with specific DMS configurations, but are also applicable to FAIMS devices. Together, these devices can be called ion mobility spectrometry devices.
Ion separation can be manifested under the influence of a strong asymmetric waveform, typically referred to as separation voltage (SV). The SV is most commonly generated using sin wave outputs. One way of accomplishing an appropriate SV field within an ion mobility cell in a DMS is described in U.S. Pat. No. 7,838,822, which is concurrently owned and hereby incorporated by reference in its entirety. This exemplary method creates the SV by applying two discrete sin waves to the mobility cell, for example a 3 MHz sin wave on the first electrode, and a 6 MHz sin wave with half the amplitude on the second electrode. The net effect with this approach is a waveform, which will be referred to herein as a FAIMS waveform, as shown in FIG. 1, and which can be utilized in either a DMS or FAIMS, A proper FAIMS waveform has the characteristic that results in an electric field in the mobility cell that is asymmetric and has a time-averaged value substantially equal to zero.
SV6 is a low amplitude, high frequency signal; SV3 is a high amplitude (approximately twice the voltage), low frequency (half the frequency) signal. (The 3 MHz sin wave is shown in green and the 6 MHz sin wave is shown in orange.) The net effect is the separation waveform shown in the burgundy trace. It should be appreciated that SV6 and SV3 are harmonics, which allows for a stable FAIMS waveform. The existing approaches to generating FAIMS waveforms have utilized DMS mobility cells having substantially less capacitance than the other capacitances in the DMS system.